Roots-type blowers have potential application in a wide variety of environments. They are relatively efficient, and can produce a wide range of delivery pressures and volumes. However, they produce a high level of noise. The high noise level produced by Roots blowers has limited their use in environments where such high noise levels are unacceptable. One such environment is providing breathing assistance to patients by means of a mechanical ventilator.
For a variety of reasons, there are instances when individuals (patients) with acute and chronic respiratory distress cannot ventilate themselves (i.e. breathe). In those circumstances, such patients require breathing assistance to stay alive. One solution is to provide those patients with a medical device called a mechanical ventilator, which assists with their breathing.
A purpose of a mechanical ventilator is to reproduce the body's normal breathing mechanism. Most mechanical ventilators create positive intrapulmonary pressure to assist breathing. Positive intrapulmonary pressure is created by delivering gas into the patient's lungs so that positive pressure is created within the alveoli (i.e. the final branches of the respiratory tree that act as the primary gas exchange units of the lung). Thus, a mechanical ventilator is essentially a device that generates a controlled flow of gas (e.g., air or oxygen) into a patient's airways during an inhalation phase, and allows gas to flow out of the lungs during an exhalation phase.
Mechanical ventilators use various methods to facilitate precise delivery of gas to the patient. Some ventilators use an external source of pressurized gas. Other ventilators use gas compressors to generate an internal source of pressurized gas.
Most ventilator systems that have an internal gas source use either constant speed or variable speed compressors. Constant speed compressors are usually continuously operating, rotary-based machines that generate a fairly constant rate of gas flow for ultimate delivery to the patient. These constant speed systems generally use a downstream flow valve to control flow of the gas to the patient, with a bypass or relief mechanism to divert excess flow that is at any time not needed by the patient (e.g. during exhalation).
Variable speed compressors operate by rapidly accelerating from a rest state to the rotational speed needed to produce the flow rate necessary during the beginning of the inhalation phase, and then decelerating to a rest or nearly rest state at the end of the inhalation phase to allow the patient to exhale.
Two types of variable speed compressor systems are typically employed in the mechanical ventilator art: piston-based systems and rotary-based systems. An example of a prior art variable speed compressor system for use in a mechanical ventilator is described in U.S. Pat. No. 5,868,133 to DeVries et al. This system uses drag compressors to provide the desired inspiratory gas flow to the patient.
Rotary compressor systems deliver the required gas flow during inhalation by accelerating the compressor rotor(s) to the desired speed at the beginning of each inspiratory phase and decelerating the compressor rotor(s) to a rest or nearly rest speed at the end of each inspiratory phase. Thus, the rotary compressor is stopped, or rotated at a nominal base rotational speed, prior to commencement of each inspiratory ventilation phase. Upon commencement of an inspiratory phase, the rotary compressor is accelerated to a greater rotational speed for delivering the desired inspiratory gas flow to the patient. At the end of the inspiratory phase, the rotational speed of the compressor is decelerated to the base speed, or is stopped, until commencement of the next inspiratory ventilation phase. Prior art systems typically use a programmable controller to control the timing and rotational speed of the compressor.
Great strides have been realized in reducing the size of mechanical ventilators. Ventilators are now available that are portable, and allow users a limited degree of autonomous mobility. Further reducing the size and power requirements of mechanical ventilators hold the potential of giving patients even greater freedom of movement, enhancing their quality of life.
Because of its relative efficiency, a Roots blower can potentially contribute to the reduction in size and power consumption of mechanical ventilators. However, heretofore it has not been possible to reduce the noise created by a Roots blower to the level that is acceptable for a mechanical ventilator.
Roots blowers use a pair of interacting rotors. Each rotor has two or more lobes. The rotors are rotated inside a housing having an inlet and an outlet. The rotors rotate with the lobes of one rotor moving into and out of the spaces between the lobes of the other. Gas is moved through the blower in chambers formed by adjacent lobes of a rotor and the adjacent rotor housing wall. These chambers will be referred to herein as “gas transport chambers.”
Noise is generated by roots blowers in a number of ways. One type of noise is caused by pulsing flow. As the rotors rotate, the gas transport chambers between the lobes of each rotor are sequentially exposed to the outlet. As each chamber is exposed to the outlet, a lobe of the mating rotor rotates into the chamber, displacing the gas in chamber to the outlet, causing a flow/pressure pulse. In the case of a pair of rotors each having two lobes, during each cycle of the blower, there are four pulses generated by the displacement of gas by the gas transport chambers. These pulses generate a substantial amount of noise.
A second type of noise is generated by a phenomenon known as “flow back.” As each rotor rotates, it inducts gas at low pressure at the inlet. This gas is generally trapped in the gas transport chambers as the rotor moves towards the outlet. When this pocket of gas initially reaches the outlet, it is exposed to higher pressure gas at the outlet. At that time, the higher pressure gas at the outlet rushes backwardly into the gas transport chamber that contains the lower pressure gas that is being delivered from the inlet.
Some attempts have been made to reduce the noise level of Roots blowers. To reduce the “pulsing” type of noise, the lobes of the rotors have been reconfigured so that they have a helical, rather than straight, shape. When the lobes of the rotors are straight, the gas flow into and out of the gas transport chamber is very abrupt. When the lobes are helical in shape, each lobe displaces gas over a larger angle of rotation. This spreads the displacement of gas over an angle of rotation, lessening the magnitude of the pressure pulse caused by the gas displacement, and reducing the noise created by the blower. However, this lobe design does not address the problem of flow back, since the relative pressure between the gas at the outlet and gas being delivered from the inlet is still the same.
Attempts have also been made to reduce flow back noise. Various kinds of channels or passages have been provided that allow some gas to flow from the outlet to the gas transport chamber prior to the time the chamber reaches the outlet, thereby increasing the gas pressure in the chamber and reducing the pressure spike that occurs when the gas in the chamber is exposed to the higher outlet pressure.
An example of a Roots blower configured with noise reducing flow-back channels is described in co-pending U.S. patent application Ser. No. 10/985,528. Although such flow-back channels are effective in reducing the level of noise, the resultant noise level may still not be sufficiently low for some mechanical ventilator applications. Accordingly, additional methods and apparatus for reducing Roots blower noise are desired.